1. Field of the Invention
The present invention is directed to a structure of a semiconductor device. More particularly, it is directed to a structure of a power semiconductor device used for power control.
2. Description of the Background Art
FIG. 10 is a cross-sectional view schematically showing the structure of a power semiconductor device in the background art. The power semiconductor device in the background art includes power elements 1, a base plate 13, an insulating substrate 4, a case 27 having electrode terminals 22 attached thereto and a cover 28.
The insulating substrate 4 consists of a ceramic substrate 6 made of alumina, aluminum nitride or silicon nitride, for example, and metal layers 5, 7 formed on both surfaces of the ceramic substrate 6. The power element 1 is joined onto the metal layer through solder 19. Further, a circuit pattern is formed on the metal layer 5. The base plate 13 made of copper or the like acts as a heat sink for heat dissipation. The metal layer 7 of the insulating substrate 4 is joined onto the base plate 13 through solder 20. The power element 1 and the insulating substrate 4 are so contained in the case 27 that the base plate 13 has an exposed surface opposite to the surface thereof for forming the insulating substrate 4.
The power element 1 is connected to the electrode terminal 22 inside the case 27 and to the circuit pattern on the metal layer 5 through aluminum wires 8. The case 27 is filled with a silicon gel 25 to cover the power element 1, the insulating substrate 4 and the base plate 13. An upper portion of the case 27 is sealed with an epoxy resin 26. Further, the cover 28 is attached to the case 27. Although not shown in FIG, 10, an external heat dissipator may be provided to the surface of the base pate 13 exposed from the case 27.
One of the electrode terminals 22 drawn to an outside of the case 27 is provided with a screw hole 24. This electrode terminal 22 is arranged in such a position that a center of a nut 29 for attaching electrode and a center of the screw hole 24 coincide with each other. An electrode terminal of an external device (not shown) is arranged on the electrode terminal 22 and a screw (not shown) is inserted in the screw hole 24 from the outside of the case 27. The screw is thereby threadedly engaged with the nut 29 buried in the case 27 for attaching electrode to establish connection and fixation of the electrode terminal of the external device to the electrode terminal 22.
In the power semiconductor device according to the background art, heat generated at the power element 1 is dissipated to the outside from the external heat dissipator (not shown) through the solder 19, the insulating substrate 4, the solder 20 and the base plate 13. The base plate 13 and the external heat dissipator are made of copper and the like having a heat conductivity of about 380 W/mK. The heat conductivity of the solders 19 and 20 ranges from 20 to 30 W/mK. The insulating substrate 4 consisting of the metal layers 5, 7 and the ceramic substrate 6 has a heat conductivity determined mainly by the heat conductivity of the ceramic substrate 6 to range from 20 to 180 W/mK. That is, the solders 19, 20 and the insulating substrate 4 have conductivities that are lower than the heat conductivities of the base plate 13 and the external heat dissipator by a large degree. Further, as the solders 19, 20 and the insulating substrate 4 are arranged directly under the power element 1, areas in each of these elements through which heat generated by the power element 1 passes are approximately the same as the area of the power element 1. For this reason, the solders 19, 20 and the insulating substrate 4 become major obstruction to heat conduction.
Alumina is the material of a high frequency of use for the ceramic substrate 6. Alternatively, another material such as aluminum nitride may be used in some cases having a heat conductivity higher than that of alumina for improvement in heat conduction of the insulating substrate 4. However, as aluminum nitride is more costly than alumina, increase in material cost has been caused.